1. Field of the Invention
The present invention relates to a communication apparatus that receives a radio signal, and more particularly, to a communication apparatus that uses an MB-OFDM (Multiband-Orthogonal Frequency Division Multiplexing) communication system.
2. Description of the Related Art
In the field of radio communication, a multicarrier transmission scheme is in the limelight as a technique for suppressing influences of frequency selective fading. In the multicarrier transmission scheme, transmission data is distributed into a plurality of carriers of different frequencies and transmitted. This narrows the bandwidth of each carrier and makes the transmission data less susceptible to frequency selective fading.
For example, in an OFDM (orthogonal frequency division multiplex) scheme, which is one of multicarrier transmission schemes, frequencies of respective carriers are set so as to be orthogonal to each other in a symbol space. At the time of transmission, a plurality of pieces of data obtained by serial/parallel-converting a serial signal at symbol periods corresponding to a rate lower than the transmission rate are allocated to their respective carriers, subjected to amplitude and phase modulation carrier by carrier, and a plurality of carriers are subjected to inverse FFT (Fast Fourier Transform). The transmission data is transformed through these processes into a time domain signal with orthogonality of the respective carriers kept on the frequency domain and transmitted. At the time of reception, the revere operation, that is, FFT transform is performed on the signal to transform the time domain signal into a frequency domain signal, the signal is subjected to demodulation corresponding to a modulation scheme of each carrier, parallel/serial-converted and the original serial signal is thereby obtained.
In an MB-OFDM communication system known as UWB (Ultra Wide Band) adopting this OFDM scheme, a frequency band of 3.1 to 10.6 GHz is divided into 14 bands, each band having a 528 MHz bandwidth, and from these bands, five groups of bands are formed, each group being made up of 3 or 2 bands. An OFDM signal is transmitted by switching between different frequency bands in each band group according to time/frequency code, that is, while realizing frequency hopping (“High Rate Ultra Wideband PHY and MAC Standard”, Ecma international standard ECMA-368 2nd Edition, December 2007, PP. 14-16).
Since the MB-OFDM communication system switches between carrier frequencies on the transmitting side by making the carrier frequencies realize hopping, the frequency of a signal received by an antenna on the receiving side also changes based on a frequency hopping pattern. When the receiving side receives a signal in such a situation, the receiving side multiplies the signal by a local signal having substantially the same frequency as a central frequency of the frequency band to which the received signal belongs via a mixer (mixing). Such mixing causes the frequency band of the received signal to be converted from the frequency band having the carrier frequency as the central frequency to a frequency band of baseband. This conversion scheme is called a “direct conversion scheme”. Here, Japanese Patent Application No. 2007-096412, which was not yet open to the public when the application for the present invention was filed, discloses that a phenomenon called “self-mixing” occurs when this direct conversion scheme is performed. The “self-mixing” refers to a phenomenon that a component of a local signal outputted from a local oscillator is mixed into a signal route of a received signal from an antenna, superimposed on the received signal from the antenna and inputted via the mixer. Once this self-mixing has occurred, a DC offset, which is an unnecessary signal component, is included in the signal outputted from the mixer. Japanese Patent Application No. 2007-096412 discloses that the MB-OFDM communication system in which the frequency of a signal received changes also needs to change the frequency of a local signal to be inputted to the mixer, and therefore the amount of the local signal component mixed into the signal route of the received signal also changes, which causes the amount of DC offset included in the signal outputted from the mixer to change as well.
FIG. 18 shows a configuration of a demodulator described in Japanese Patent Application No. 2007-096412. Japanese Patent Application No. 2007-096412 discloses a technique for removing a DC offset of a signal outputted from a mixer while also coping with a variation in the amount of DC offset caused by a variation in the frequency of the local signal outputted from the local oscillator in a region where the signal received via the antenna is an analog signal. To be more specific, DC offsets generated in differential signals outputted from the mixer are stored in a capacitor. The above described document discloses the technique of turning ON a switch after the capacitor in synchronization with the switching of a hopping frequency at a guard interval, short circuiting the differential signal line and thereby removing the DC offset seen as the differential signal.
Furthermore, for example, Japanese Patent Laid-Open No. 2006-80689 discloses a technique of removing such unnecessary signal components. A communication apparatus having a successive reception mode and intermittent reception mode repeatedly starts and stops reception. An excessive response occurs every time a reception starts. If a signal is received when an excessive response occurs and baseband processing is performed, the reception error rate deteriorates. The technique described in Japanese Patent Application No. 2007-096412 acquires data indicating this excessive response beforehand and stores the data in a storage section and corrects, when a reception starts, the waveform of the received signal using the data indicating the excessive response stored in the storage section.
The present inventor discovered that above described Japanese Patent Application No. 2007-096412 involves the following problems. Although Japanese Patent Application No. 2007-096412 claims that it is possible to remove a DC offset while coping with a variation in the amount of the DC offset caused by a frequency variation of the local signal, there are cases where the technique described in Japanese Patent Application No. 2007-096412 cannot completely remove the DC offset generated in the received signal and the DC offset remains (the reason will be described later). The signal including the remaining DC offset is amplified by an amplifier, then converted to a digital signal by an AD converter, subjected to processing such as FFT (Fast Fourier Transform) and then subjected to various types of digital processing such as Viterbi decoding that carries out error correction.
Here, the MB-OFDM communication system performs processing called “overlap-and-add” (hereinafter, referred to as “overlap and add processing” in the present Specification) on the received signal converted to a digital signal. “Overlap-and-add” adds a component near the rearmost portion of a signal included within a certain predetermined time window to a component near the start of the signal included in such a time window. In radio communication, there are not only routes through which a signal directly arrives at a reception antenna from a transmission antenna but also routes through which a signal is repeatedly reflected at some midpoints and arrives at the reception antenna (so-called multipath). In this case, signals including the same information arrive at the antenna on the receiving side at different timings. The signal arriving at the reception antenna after repeating reflection arrives with a delay compared to the signal directly arriving at the reception antenna from the transmission antenna. On the other hand, since the delayed signals also include information to be received, it may be more efficient from the standpoint of receiving power for the receiving side to also regard the delayed signals as the reception targets. However, since the degree of time window with which FFT is performed on the received signal on the receiving side is predetermined, the receiving side cannot directly apply FFT to signal components so delayed as to go beyond the time window within which FFT is applied. This is because extending the predetermined time window for applying FFT according to a delay signal results in a problem that the frequency of the signal obtained through FFT is shifted from the frequency that should originally be acquired, that is, the frequency of the signal used on the transmitting side. The circuit on the receiving side is designed to be able to perform processing on signals of frequencies that should originally be acquired, and cannot handle signals of changed frequencies.
However, considering receiving power of a signal, the delay component of the received signal is also preferably handled as an object to be received. Therefore, MB-OFDM performs the above described overlap-and-add, that is, overlap and add processing. By adding a signal component so delayed as to go beyond a time window within which FFT can be applied to a signal component included near the start of the time window, it is possible to efficiently gain receiving power including the delay component of the received signal.
However, as described above, the technique according to Japanese Patent Application No. 2007-096412 may not be able to completely remove DC offsets depending on the situation. The reason is as follows. As shown in FIG. 18, the demodulator according to Japanese Patent Application No. 2007-096412 turns ON the switch after the capacitor during a guard interval period for switching between frequencies of local signals and causes the differential signal lines to be short circuited to thereby reduce the potential difference between the differential signal lines to 0. By so doing, the DC offsets when seen as the differential signals are removed. On the other hand, according to Japanese Patent Application No. 2007-096412, a separate switch is also provided before the frequency conversion section. The reason is as follows: When reducing the potential difference between the differential signal lines by turning ON the switch after the capacitor during the guard interval period to 0, the switch before the frequency conversion section is also turned ON for such a guard interval period so as to prevent noise from the antenna and low-noise amplifier from entering sections from the frequency conversion section and subsequent sections for a period of charging/discharging the capacitor. When a noise component is inputted to the capacitor for the period of charging/discharging the capacitor, the amount of charge stored in the capacitor changes and the potential difference between the two differential signal lines can no longer be reduced to 0 during the guard interval period. This is intended to avoid such a problem. Therefore, the technique in Japanese Patent Application No. 2007-096412 turns ON the switch before the frequency conversion section first, then turns ON the switch after the capacitor, then reduces the potential difference between the differential signal lines to 0, turns OFF the switch after the capacitor and then turns OFF the switch before the frequency conversion section.
However, since signals handled by the switch after the capacitor shown in FIG. 18 are baseband signals, this is a switch using a large-size transistor, whereas the switch before the frequency conversion section is a switch applicable to a high-frequency signal. High-frequency switches need to use small-sized transistors. This is because using a large-sized transistor would cause the component of parasitic capacitance to increase and interfere with the operation of the demodulator applicable to wideband signals. Therefore, the switch before the frequency conversion section in FIG. 18 includes ON resistance of not negligible magnitude and even if such a switch is turned ON, the two differential signal lines are not completely short circuited. In such a case, even if such a switch is turned ON, it is not possible to completely prevent noise from entering parts from the frequency conversion section onward.
This is because when ON resistance of not negligible magnitude exists even when the switch is ON, not only a signal component flowing through the switch but also a signal component flowing through the frequency conversion section is produced. Therefore, as described above, the technique according to Japanese Patent Application No. 2007-096412 turns OFF the switch after the capacitor during the guard interval period and then turns OFF the switch before the frequency conversion section, but since the two differential signal lines are not completely short circuited, when noise from the antenna or low-noise amplifier enters the demodulator in FIG. 18 after the switch after the capacitor is turned from ON to OFF, such noise also enters sections from the frequency conversion section onward. In such a case, the potential difference between the two differential signal lines is not reduced to 0 and reception of symbols including the information to be received is started. As explained so far, depending on the situation, there are cases where the technique according to Japanese Patent Application No. 2007-096412 cannot reduce the potential difference between the differential signal lines to 0 or completely remove DC offsets.
This means that a DC offset remains in the received signal to be subjected to the above described overlap and add processing. For example, performing the overlap and add processing in this condition results in a problem that the DC offset in the portion of the signal to be added increases. Moreover, since this remaining DC offset is amplified by an amplifier before being converted to a digital signal, the amount of DC offset increased through the overlap and add processing cannot help but further increase. Of course, the DC offset corresponding to the signal not subjected to the overlap and add processing is also amplified by the amplifier. Failure to remove the remaining offset has an adverse influence on the posterior processing such as FFT and decoding of error correcting code. Thus, the MB-OFDM communication system needs to appropriately remove, in the digital region, the DC offset which has not been removed in the analog region.